The invention pertains to a method for the oriented solidification of molten silicon to form an ingot in a bottomless crystallization chamber with a cooling body supporting the ingot, the cooling body being lowered by relative motion with respect to the crystallization chamber at a rate dependent on the supply of additional silicon and the solidification rate. The flat surface of a seed body with a columnar crystal structure, which initially at least almost completely closes off the crystallization chamber, is laid on the surface of the cooling body and is superficially melted by the molten silicon. The cooling body supporting the seed body and the already solidified part of the ingot is then removed by relative motion from the crystallization chamber.
Ingots of this type are used as starting material for the production of, for example, photovoltaic elements such as solar cells. For this purpose, the ingots are sliced into extremely thin wafers with a thickness of less than 500 .mu.m and then subjected to the required further processing. During oriented solidification, either so-called "fringe" crystals (in a columnar arrangement) or monocrystals can be produced. The production of single crystals by the Czochralski method, for example, is thus quite expensive.
Studies have shown that the efficiency with which solar energy is converted to electricity depends very strongly on the microstructure of the silicon. The efficiency of a glassy structure, that is, of amorphous silicon, is extremely low, but it increases with the size or columnar arrangement of the crystals, reaching values of approximately 15% or more in so-called monocrystalline cells. The reason for this behavior is considered to be the presence of impurities at the grain boundaries, which lead to the recombination of charge carriers. These relationships are described in the book by Bergmann and Schaefer entitled Lehrbuch der Experimentalphysik [Textbook of Experimental Physics], Vol. 6, Solids, Verlag Walter de Gruyter, Berlin and New York, 1992, pp. 551-553, so that no further discussion in required here.
U.S. Pat. No. 4,572,812 discloses melting silicon in a square, bottomless, cold crucible, made of vertically arranged, palisade-like, water-cooled bars, surrounded by an induction coil, and drawing solid ingot continuously from the melt in a kind of continuous casting process by a support plate, which draws the ingot down continuously. This support plate consists of graphite, and it is heated by the induction coil. One of the reasons why the plate is heated is to preheat the lump silicon to a temperature sufficient for electric current to flow through it. Once this point has been reached, the silicon can then be heated further by the flow of inductive energy until it has completely melted. The ingot thus produced has a cross section of 25.times.25 mm.sup.2 and a length of 17 cm. Although it is stated that the ingot is intended to have a coarsely crystalline structure, this is a relative statement which offers no indication of the absolute size of the crystals, especially in view of the small ingot cross section. Nor is the orientation of any columnar crystals which may be present disclosed.
From the article by Kaneko et al. entitled "Cold Crucible Induction Casting of Semiconductor Silicon", published in the Proceedings of the Sixth International Iron and Steel Congress, 1990, Nagoya, ISIJ, pp. 254-259, it is known that a silicon melt can be produced in a bottomless, water-cooled, cold crucible and that the ingot being formed can be drawn continuously from the cold crucible. The crucible in question has a square cross section with inside dimensions of 80.times.80 mm.sup.2. The stated efficiency of the energy conversion by the end product is 13.7%.
In cold crucibles of this type, the molten silicon does not become contaminated; specifically, it does not take up any oxygen, especially since the process is carried out under vacuum or under a shielding gas. Because the cold crucible, which is slotted in the axial direction, is heated by induction, the possibility of contact between the molten silicon and the walls of the cold crucible is completely excluded. No details are provided concerning the bottom of the crucible, which can be lowered. Without any special measures, however, the crystals which are formed are thin and columnar, with a small cross sections, which would probably explain the low efficiency given for the end product.
The article by Servant et al. entitled "Grain structure of silicon solidified from an inductive cold crucible", published in Materials Science and Engineering, Vol. A173, 1993, pp. 63-66, furthermore, presents a method of the general type described above in which a silicon bar is inserted into the crucible and melted before the continuous addition of lump silicon begins. Both round and square cold crucibles are disclosed; the round cold crucible is said to have an inside diameter of 102 mm, whereas the square cold crucible is said to have inside dimensions of 60.times.60 mm.sup.2. Concerning crystal structure, it is stated that it is possible to distinguish three zones, namely, an outer quenching zone with very small columnar crystals with diameters of 0.1 mm; a transition zone with a larger grain diameter of 2-3 mm; and a central zone with larger grain diameters of 3-4 mm, or, when direct cooling is used, with diameters of 5-8 mm. Thus the grain size spectrum within the ingot cross section is extremely wide, which is as unsuitable as could be imagined for the production of high-efficiency solar cells. The reason given for this unsuitability is the direction of grain growth: initially, the grains grow in a radial direction from outside to inside, whereas, in the center of the ingot, they grow almost vertically. This difference is attributable to an extremely deep melt pool, the depth of which, in the center, is 0.40-0.42 times the diameter of the ingot. As a result, the boundary between the liquid phase and the solid phase is in the shape of a paraboloid, which is also shown in the drawing. Normally, the grains always grow in the direction normal to the solid/liquid phase boundary.
Specific types of problems are encountered in the attempt to produce ingots of silicon in cold crucibles. These are attributable to the very poor thermal conductivity and lack of coupling between the electrically nonconducting silicon and the inductive heating sources at temperatures below approximately 700.degree. C. The silicon does not become sufficiently coupled inductively to an induction coil until the temperature reaches about 1,000.degree. C. Conventional doping cannot change much in this situation. At these temperatures, however, the silicon is extremely reactive, readily taking up substances with which it comes in contact.
U.S. Pat. No. 4,572,812 also describes problems concerning electrical conductivity. To achieve good electrical conductivity, the first possibility given is to bring a graphite heating rod into contact with the semiconductor material. When this is done, however, the energy is transferred centrally, in an almost pointwise or at best linear manner; this is associated with pronounced overheating, and the heat can dissipate only gradually by thermal conduction through the poorly conductive silicon. The semiconductor material also becomes contaminated with traces of graphite. The second possible solution is to use the bottom plate of graphite as a heating element by coupling it to the induction coil. This is associated right away, however, with two disadvantages. First, precisely during the starting phase, the bottom plate cannot be used to initiate the crystallization process; and, second, if a seed crystal plate were to be laid on the bottom plate, it would melt completely and thus lose its crystal structure, which is vital to the entire drawing process. Finally, the bottom area of the ingot would also be contaminated with traces of graphite in this case, so that part of the ingot would have to be discarded. The semiconductor ingot is said to have a cross section of 25.times.25 mm.sup.2, which is relatively small. The problems of external heating by coupling graphite to the silicon, however, increase as the cross section of the ingot becomes larger, because the thermally affected zone is limited and thus strong radial temperature gradients develop.
It also known, finally, that the silicon melt can be produced in a hot crucible made of quartz. Two disadvantages are associated with this approach: first, oxygen will migrate from the quartz crucible to the molten silicon, which is extremely undesirable; and, second, the quartz crucible will be destroyed Then the silicon solidifies. This destruction is attributable to the fact that silicon has a linear expansion rate of 9.6% at the solidification point, which inevitably leads to the destruction of the quartz crucible. Therefore, it is necessary to use a new quartz crucible for each batch. This means that production costs are extremely high.
EP 0 021 385 A1 of the general type in question describes how a polycrystalline semiconductor rod of silicon, which has solidified with longitudinal orientation, can be produced by a continuous process in an inductively heated crystallization chamber of graphite, which is open at the bottom, that is, in a hot crucible. This rod is supported on a plate-shaped cooling body of graphite. The oriented solidification is brought about by using a plate-shaped seed crystal, which is laid on the cooling body, the horizontal dimensions of the crystal corresponding to the cross-sectional area of the silicon rod. Nothing is stated concerning the cross-sectional areas of the individual columnar crystals. The crystallization chamber, consisting of graphite, serves as a susceptor for the inductive heating power; that is, the heat or melting heat is supplied to the content of the crystallization chamber by the extremely hot chamber or crucible wall in the direction radial to the axis of the silicon rod. To prevent undesirable reactions of the graphite components (crystallization chamber and cooling body) with the silicon, the inside surfaces of these parts must be coated with a protective melt of a material inert with respect to silicon in a thickness of approximately 10-20 .mu.m. This protective melt has high surface tension and must be maintained throughout the drawing process.
The protective melt, the temperature of which must be kept below the melting point of silicon (1,410.degree. C.), should contain preferably calcium fluoride. Calcium and all other alkaline-earth metals, however, are readily soluble in silicon, according to, for example, the reaction: EQU 2CaF.sub.2 +Si.fwdarw.SiF.sub.4 +2Ca.
SiF.sub.4 is a gas, some of which leads to the formation of bubbles in the silicon, which is extremely undesirable, while some escapes and reacts with the moisture in the atmosphere to form the extremely toxic compound HF: EQU 2H.sub.2 O+SiF.sub.4 .fwdarw.SiO.sub.2 +4HF.
The presence of silicon dioxide as slag in the ingot, however, means the end of all semiconductor technology.
A vertical temperature gradient, furthermore, must also be maintained within narrow limits inside the crystallization chamber, so that the silicon rod, as it solidifies inside the crystallization chamber, will not rupture the chamber as a result of its high coefficient of expansion. This gradient is very difficult to control by means of automatic control technology. Because the cooling body, consisting of graphite, can dissipate only relatively small amounts heat in the axial direction, the crystal rod must be cooled by directing jets of cooling gas at it radially from ring nozzles, but this can be done only after the starting phase, i.e., after the phase during which the seed crystal plate is still located inside the crystallization chamber.
During the especially important starting phase, therefore, the axial heat dissipation is very slow, so that columnar crystals (fringe crystals) of relatively small cross section are obtained at the edges and at the ends of the silicon rod; these small crystals then propagate through the silicon rod. Columnar crystals of small cross section, however, lead to end products (solar cells) of poor efficiency. By directing jets of cooling gas against the silicon rod, furthermore, the effect is again produced in ingots of large cross section or large diameter that, because of the poor thermal conductivity of silicon, strong horizontal temperature gradients develop, as a result of which the solid/liquid phase boundary becomes increasingly parabolic--an effect which increases as the rod becomes longer.
As a result, the original orientation of the columnar crystals, namely, the orientation parallel to the axis, is lost again, and extremely small individual crystals are formed. In addition, the atmosphere inside the apparatus is disturbed. According to the examples, the ingot cross sections are limited to 30.times.30 mm.sup.2 or 100.times.100 mm.sup.2.